1. Technical Field
The present invention relates to a printer that has a vacuum platen that holds the recording paper to the platen surface defining the printing position while the paper is conveyed, and relates more particularly to a recording paper transportation path structure that guides the recording paper delivered to the platen surface through the nipping part of a paper feed roller and a paper pressure roller.
2. Related Art
Holding the recording paper tight to the platen surface that defines the printing position in a printer, such as an inkjet printer, and assuring a precise platen gap is important as a means of assuring print quality. In order to convey while holding the recording paper to the platen surface, a vacuum platen that pulls the recording paper to the platen surface by means of air suction while the paper is conveyed is used. More particularly, because continuous recording paper such as roll paper and fanfold paper typically has a curl or crease, a vacuum platen is preferably used to forcibly hold the recording paper flat. Printers having a vacuum platen are taught in Japanese Unexamined Patent Appl. Pub. JP-A-2001-212946 and Japan Patent No. 3864791, for example.
If the part of the recording paper that is delivered from the nipping part of the paper feed roller and paper pressure roller to the platen surface of the vacuum platen is fed in a direction lifting away from the platen surface, a gap is produced between the recording paper and the platen surface, a vacuum condition cannot be produced by air suction therebetween, and it may not be possible to pull the recording paper to the platen surface. It is therefore preferable to direct the paper feed direction of the recording paper that is fed from the nipping part between the rollers to the platen surface at an angle causing the paper to approach the platen surface instead of parallel to the platen surface, and thereby feed the part of the recording paper advanced from the nipping part so that the paper is pushed to the platen surface.
However, problems such as described below can occur when this type of recording paper transportation path structure is used.
More specifically, as shown in FIG. 7, in order to feed the recording paper 100 in a direction pushing it toward the platen surface 102 of the vacuum platen 101, a step A must be provided between the nipping part 105 of the rollers 103 and 104 and the platen surface 102. If the nipping part 105 and the platen surface 102 are at the same height, the recording paper 100 advanced from the nipping part 105 is guided from the edge 102a of the platen surface 102 while being supported by the platen surface 102. However, when there is a step A, the part of the recording paper 100 fed from the platen surface 102 contacts the platen surface at a position on the downstream side in the transportation direction from the edge of the platen surface as indicated by dot-dash line B.
When such a step is provided, the distance until the recording paper 100 fed from the nipping part 105 is supported by the platen surface 102 increases, and the part of the recording paper 100 passed from the nipping part 105 to the platen surface 102 is conveyed with a sag imparted thereto by the suction of the vacuum platen 101.
As a result, when a curled or creased part of the recording paper 100 is advanced from the nipping part 105, it sags greatly into the gap 110 between the nipping part 105 and the edge 102a. If the recording paper 100 sags greatly, the part of the recording paper 100 conveyed over the platen surface 102 becomes shorter than the length of the part of the recording paper 100 fed from the nipping part 105 by the amount of this sag. As a result, the dot pitch of the inkjet head may vary and print quality may drop because not enough paper is advanced and there is a drop in the paper feed precision.
Particularly when using recording paper such as fanfold paper, paper stiffness differs greatly at different parts along the paper length. For example, because fanfold paper is particularly weak at the folds, when the folded part is passed from the nipping part to the platen surface, the folded part bends and sags deeply into the gap 110 between the nipping part and the platen surface, may become stuck between the platen surface 102 and the roller 103, and a paper jam may result.